Technical Field
The invention relates generally to apparatus for preparing a production well such as a gas or oil well. More specifically, the invention relates to a gravel packing system used in a well to place gravel in casing perforations of the well at a formation site.
Discussion of Related Art
An oil well borehole which is being prepared for oil and/or gas production generally includes a steel casing supported by a cement casing in the annulus around the steel casing. The cement casing isolates two or more zones such as, for example, a production zone from brine. A number of perforations are formed in the casings at the formations, thus providing fluid communication between the formation and the well. A production string wellstring provides a fluid conduit through which the oil or gas travels to the surface. A portion of the production string opposite the casing perforations is referred to as the screen. The screen is made of tubing with numerous holes formed in the tubing wall. Wire is then wrapped around the tubing so as to achieve a desired mesh which permits the formation products to flow up the production string but blocks undesired deposits entrained in the oil or gas.
A serious problem encountered during extraction is the presence of formation sand in the product. Because of the high fluid pressures involved, there is a sandblasting effect on the screen which can quickly lead to premature weardown of the screen and tubing.
A common technique used to overcome this blasting effect of the formation sand is to pack gravel in the casing perforations and in the annulus around the screen. The gravel acts as a trap which blocks the formation sand from reaching the screen but which permits permeability for the product medium such as oil to flow through to the production string.
The gravel is mixed with water and pumped as a slurry down the well to the formation site. The gravel must be effectively packed to prevent voids. When packed under pressure the slurry dehydrates, with the fluid being returned to the surface via a washpipe.
The gravel packing process is carried out using a packer apparatus and a service tool. Generally, the packer is an apparatus which in normal use is placed in the well and directs the slurry to flow to the desired location for packing. The packer performs this task by separating the annulus between the string and casing into two sealed off regions, the upper annulus above the packer and the lower annulus which is below the packer. The packer is provided with a plurality of slips which can be hydraulically actuated to bite into the steel casing to support or set the packer in the well hole. A plurality of packer sealing elements are compressed and expanded radially outwardly to seal off the upper annulus from the lower annulus.
The hydraulic actuation of the packer is effected by the use of another tool called the service tool which may also be referred to as a running tool or cross-over tool. The service tool is screwed into the packer and both tools are run into the well with a workstring. The service tool provides a conduit via tubing for hydraulically setting the packer and provides cross-over ports for carrying the slurry from the tubing over into the lower annulus through openings or squeeze ports in the packer housing.
In normal use the service tool is removed from the well after the packing operation is completed and the packer remains set in the well. After the service tool is removed the production string can be run into the well and extraction of the formation products is carried out.
The packer and service tool assemblies known heretofore, however, have numerous drawbacks and very undesirable limitations. For example, because the service tool and packer are screwed together, in order to remove the service tool it must be unscrewed from the packer via the workstring. This procedure requires the application of high torque levels on the workstring in order to rotate and back out the service tool from the packer. This is particularly difficult in highly deviated (curved or nonvertical) wells wherein the torque applied to the workstring is prohibitive.
Another problem with the known packers and service tool is the tendency for the packer assembly to relax when the setting pressure is removed, thus reducing the effectiveness of the packer seal elements and the slips which support the packer in the casing.
Another significant problem is that when it becomes necessary to perform a run to retrieve the packer, the packer must be pulled out with a tremendous force necessary to free the packer from the casing due to the high slip load.